The present invention relates to a data receiving apparatus for use with digital mobile telephones or the like.
FIG. 1 is a block diagram showing the structure of a conventional data receiving apparatus. In FIG. 1, reference numeral 1 represents an equalizer for removing the influence of frequency selective fading from a received signal. Reference numeral 2 represents a discriminator connected to the equalizer 1 for decoding the received signal with the influence of frequency selective fading removed, to obtain data of the received signal. The data decoded by the discriminator 2 is thereafter subject to voice processing. Reference numeral 3 represents a coefficient renewing unit coupled between the equalizer 1 and discriminator 2, for renewing the coefficient of the equalizer 1 in accordance with an error signal detected when the discriminator 2 decodes the received signal to obtain the data.
FIG. 2 shows the detailed structure of the equalizer 1. In FIG. 2, reference numeral 11 represents a delay element. A plurality of delay elements 11 are provided for calculation of received signals sampled in the past. Reference numeral 12 represents multipliers each for multiplying the past received, sampled signal by a coefficient set by the coefficient renewing unit. Reference numeral 13 represents an adder for adding together the outputs of the multipliers 12.
The operation of the conventional data receiving apparatus constructed as above will be described. In FIG. 1, when a received signal is inputted to the equalizer 1, the received signal and past received, sampled signals are multiplied by a predetermined coefficient and added together, the result being outputted to the discriminator 2. The discriminator 2 decodes the inputted signal to obtain decoded data, by selecting a transmission signal nearest, on a phase plane, to the inputted signal from the equalizer 1 and by performing a predetermined modulation scheme such as Gaussian-filtered Minimum Shift Keying. The discriminator 2 also obtains an error signal representing a difference between its input and output signals, and outputs it to the coefficient renewing unit 3. The coefficient renewing unit 3 renews the coefficient of the equalizer 1 to be used for the received signal sampled at the next step by using the following equation (1), in accordance with the error signal outputted from the discriminator 2, so as to make the error signal take a zero value assuming that there is no change in the state of transmission line. This renewal is performed using a coefficient renewal algorithm such as a least mean square (LMS) algorithm described in Widrow B., et al., "Adaptive noise Canceling: Principles and Applications", Proc. IEEE, 63, pp. 1692 to 1716 (1975). EQU K.sub.i+1 =(K.sub.i +.alpha..multidot.r) (1)
where K.sub.i+1 is a coefficient at the present step, K.sub.i is a coefficient at the last step, .alpha. is a constant, and r is an error signal.
In this manner, if the state of the transmission line does not change, the conventional data receiving apparatus can obtain correct decoded data while removing the influence of frequency selective fading.
However, if the center frequency of the received signal has any shift, the state of transmission line changes with time so that the discriminator 2 of the conventional data receiving apparatus cannot correctly decode the received signal and obtain its data.